1. Field of the Invention
This invention relates to an electrochemical method and apparatus for the synthesis of nitrogen fertilizers including ammonium nitrate, urea, ammonia, and urea-ammonium nitrate. In particular, the invention relates to an apparatus and method whereby (1) a nitrogen source is utilized to produce ammonium nitrate; (2) a nitrogen source and a carbon source are reacted using liquid electrolyte at low temperature or solid electrolyte at high temperature to form urea; (3) a nitrogen source and a hydrogen-equivalent source are reacted to generate ammonia; and (4) a nitrogen source and carbon source are reacted to produce urea-ammonium nitrate.
2. Background of the Invention
Ammonium nitrate (AN, 34% N), urea (46% N), ammonia (82% N) and urea-ammonium nitrate (UAN, 28%˜32% N) are widely used high nitrogen-content fertilizers. Methods for industrial production of these fertilizers are mainly based on the Haber process, which involves the heterogeneous reaction of nitrogen and hydrogen on an iron-based catalyst at high pressure (for example, 200-300 bar) and high temperature (for example, 430° C.-480° C.) to produce ammonia as follows:N2(g)+3H2(g)2NH3(g)  (Rea. 1)
The conversion to ammonia, shown in Reaction 1, is limited by thermodynamics. The gas volume decreases as the reaction progresses. Hence, very high pressure must be used to drive the ammonia synthesis reaction to the right in Reaction 1, which is in the direction of ammonia gas. Carrying out ammonia synthesis at very high pressure is also necessary to prevent decomposition of synthesized ammonia into nitrogen and hydrogen and to provide practical reaction rates. In addition, Reaction 1 is exothermic, and ammonia formation increases with decreasing temperature. Reducing the temperature, however, undesirably reduces the rate of the reaction. Therefore, an intermediate temperature is selected such that the reaction proceeds at a reasonable rate, but the temperature is not so high as to drive the reverse reaction. The equilibrium conversion of hydrogen gas and nitrogen gas to ammonia is generally only on the order of 10%˜15%. Low conversion efficiencies give rise to cost-intensive, large-scale chemical plants and costly operating conditions required to commercially produce hundreds to thousands of tons per day of ammonia in an ammonia synthesis plant.
Ammonium nitrate (AN) is produced via the acid-base reaction of ammonia with nitric acid according to the equation:NH3+HNO3→NH4NO3  (Rea. 2)
Industrial nitric acid is manufactured by the high-temperature catalytic oxidation of ammonia. This process typically consists of three steps: first, ammonia is reacted with air on PtIr alloy catalyst at around 750° ˜800° C. to form nitric oxide according to the following reaction:4NH3+5O2→4NO+6H2O  (Rea. 3)Next, nitric oxide is oxidized to nitrogen dioxide and its liquid dimer as follows:2NO+O2→2NO2N2O4.  (Rea 4)And, finally, the nitrogen dioxide/dimer mixture is introduced into an absorption process using water in accordance with the following reaction:3NO2+H2O→2HNO3+NO  (Rea. 5)In the first step, the oxidation of ammonia to nitric oxide proceeds in an exothermic reaction with a range of 93% to 98% yield. Reaction temperatures can vary from 750° C. to 900° C. Higher temperatures increase reaction selectivity toward NO production. Reaction 3 is favored by low pressures. In the second step, Reaction 4 is slow and highly temperature- and pressure-dependent. Operating at low temperatures and high pressures promotes maximum production of NO2 within a minimum reaction time. The final step, Reaction 5, is exothermic, and continuous cooling is therefore required within the absorber. As the conversion of NO to NO2 is favored by low temperature, this reaction will take place significantly until the gases leave the adsorption column.
The commercial production of urea is based on the reaction of carbon dioxide and ammonia at high pressure (for example 140 bar) and high temperature (for example 180° ˜185° C.) to form ammonium carbamate (Reaction 6), which is subsequently dehydrated into urea and water (Reaction 7):2NH3+CO2→NH2COONH4  (Rea. 6)NH2COONH4→NH2CONH2+H2O  (Rea. 7)
Reaction 6 is fast and highly exothermic and goes essentially to completion under normal processing conditions, while Reaction 7 is slow and endothermic and usually does not reach thermodynamic equilibrium under processing conditions. The degree to which Reaction 7 proceeds depends on, among other factors, the temperature and the amount of excess ammonia used. Increasing temperature and the NH3:CO2 ratio could increase the conversion of CO2 to urea.
Different urea production technologies basically differ on how urea is separated from the reactants and how ammonia and carbon dioxide are cycled. Refinements in the production technology are usually concentrated on increasing CO2 conversion, optimizing heat recovery, reducing utility consumption, and recovering residual NH3 and urea from plant effluents.
Ammonium nitrate and urea are used as feedstocks in the production of urea-ammonium nitrate (UAN) liquid fertilizers. Most UAN solutions typically contain 28%, 30% or 32% N, but other customized concentrations (including additional nutrients) are produced. The addition of corrosion inhibitors or the use of corrosion-resistant coatings allows carbon steel to be used for storage and transportation equipment for the solutions.
Continuous and batch-type processes are used, and, in both processes, concentrated urea and ammonium nitrate solutions are measured, mixed, and then cooled. In the continuous process, the ingredients of the UAN solution are continuously fed to and mixed in a series of appropriately sized static mixers. Raw material flow as well as finished product flow, pH, and density are continuously measured and adjusted. The finished product is cooled and transferred to a storage tank for distribution. In the batch process, the raw materials are sequentially fed to a mixing vessel fitted with an agitator and mounted on load cells. The dissolving of the solid raw material(s) can be enhanced by recirculation and heat exchange as required. The pH of the UAN product is adjusted prior to the addition of the corrosion inhibitor.
As described above, the production of high-nitrogen fertilizers involves multi step reactions and is strongly limited by the Haber process. The equilibrium conversion of hydrogen gas and nitrogen gas to ammonia in the Haber process is generally only on the order of 10%˜15%. Such low conversion efficiencies give rise to cost-intensive, large-scale chemical plants and costly operating conditions required to commercially produce hundreds to thousands of tons per day of ammonia in an ammonia synthesis plant. Therefore, it is of industrial interest to develop simplified approaches for the production of high-nitrogen fertilizers, especially at small to middle scales. A one-step process can convert carbon sources, nitrogen sources, and/or hydrogen sources to the high-nitrogen fertilizer at decreased pressure and/or temperature has the potential to meet such requirements of small- to middle-scale production of high-nitrogen fertilizers.
Only recently has the feasibility of using electrochemical processes for urea synthesis been investigated. The most obvious advantages of electrochemical processes over traditional processes mentioned above include (1) simplified process complexity since a one-step process is likely, (2) simplified operation conditions since electrochemical reaction could be run even at room temperature and atmospheric pressure, and (3) decreased system volume and size. Several challenges exist, however, for industrial consideration of the reported electrochemical process for the preparation of urea. One of these challenges is the high cost of the nitrogen source, as nitrite and nitrate are typically employed. Another of these challenges is the high cost of the hydrogen source, as more hydrogen gas is required because of the use of above-mentioned nitrite or nitrate compared to the use of a low-valance nitrogen source such as nitrogen gas or nitric oxide. Commercialization of the process is also deterred by the fact that the current efficiency for urea formation has been low and the process has not been optimized. The process involves the reduction of carbon dioxide and nitrogen-containing compounds. The two reactions corresponding to (1) electrochemical reduction of CO2 and (2) electrochemical reduction of nitrogen-containing compounds are thermodynamically and kinetically different. To promote urea formation, the rates of these two reactions must be precisely controlled, and this requires developing electrocatalysts, selecting electrolytes, controlling the composition of the reactants and the feeding rates of the reactants and electrolytes, choosing current or potential control mode, and implementing temperature control. Additionally, the structures of the electrodes and the electrochemical cells need to be optimized to improve process efficiency and decrease process complexity. Finally, for commercial production, stacks comprising several electrochemical cell units must be designed, and control systems for these stacks need to be developed.
Recently, attention has been drawn to the removal of CO2 and nitrogen oxides from the environment, as it is conjectured that these compounds contribute to serious problems, including the “greenhouse effect” and acid rain.
The present invention includes electrochemical processes for the production of nitrogen fertilizers including ammonium nitrate, urea, ammonia, and urea-ammonium nitrate, using cost-effective sources of carbon and hydrogen or hydrogen equivalent such as carbon monoxide. One embodiment of the present invention is a low-temperature and low pressure electrochemical process for the production of a nitrogen fertilizer without the need for a hydrogen input. Another embodiment is an electrochemical process for urea production using a cost-effective nitrogen source, carbon sources and a low-cost hydrogen equivalent rather than high-purity hydrogen as required for the Haber and other processes. Another embodiment is an electrochemical process for ammonia production using a cost-effective nitrogen source and a low-cost hydrogen equivalent. Another embodiment is an electrochemical process for the production of urea-ammonium nitrate using cost-effective sources of nitrogen and carbon. Another embodiment utilizes greenhouse gases in the electrochemical process for the production of nitrogen fertilizers. The present invention also encompasses electrochemical reactors and reactor components developed specifically for the above-described embodiments.